Create a `specification-change` skill

A skill to help with updating the specifications. It should explain:
- The different states specifications can be in e.g. draft, candidate, final etc.
- The types of changes that are allowed in each state
- How to create a new specification
- The writing style to use
- The set of files to consider e.g. both Markdown and JSON
- How to write and run the appropriate tests

Perhaps most of this is just regular documentation at `specification/docs` that is referenced by a skill. That way, it can also be discovered by humans easily.

The skill should specifically mention some things that need to be considered that are non-obvious including:
- Make sure you update the rules governing the structure of catalog documents, including the "allowed schemas" in `common-types.json` that can be referenced from component and function schemas.

This comes from @gspencergoog 's idea in https://github.com/a2ui-project/a2ui/pull/1629#discussion_r3424979613